1. Field of the Invention
The present invention relates to a continuous process for producing a propylene-ethylene block copolymer. More particularly, it relates to a continuous process for producing a propylene-ethylene block copolymer having excellent characteristics of cold shortness and high impact strength which are the same or superior to those of a propylene-ethylene block copolymer obtained by a batch system.
2. Description of the Prior Art
It has been well-known to produce a propylene-ethylene block copolymer in order to improve an impact strength and a cold shortness over those of a homopolymer of propylene. In general, propylene is polymerized in the presence of a Ziegler-Natta catalyst composed by an organoaluminum compound and titanium trichloride or a titanium chloride supported by a magnesium compound in an inert liquid hydrocarbon or liquid propylene as a diluent in the first step; and copolymerizing propylene with ethylene in the presence of the propylene polymer produced in the first step to produce a block copolymer in the second step.
The polymerization process can be a batch process and a continuous process. The batch process causes certain disadvantages because of relatively low productivity and economical disadvantage and indispensable dispersion of qualities of the product in runs of the production. On the other hand, the continuous process does not cause such disadvantages to be advantageous as an industrial process. In view of the quality of the product, the following disadvantages are found.
When the first step and the second step are connected in series in the two step polymerization and propylene is polymerized in the first step and propylene and ethylene are copolymerized in the second step as the continuous process, the resulting block copolymer has inferior cold shortness and impact strength in comparison with those of the batch process and an ultimate elongation is lowered and fish-eyes are formed in the product. As a main reason why such disadvantages are caused, the following reason may be considered.
When two or more reactors are connected in series to polymerize by the continuous process, it is considered to be the complete mixed condition in each reactor and to result in certain distribution of the residence time of the catalyst or the polymer containing the catalyst in each reactor. When the residence time of the catalyst is short, the polymer having relatively low productivity of the polymer per unit of the catalyst (referring to as catalyst efficiency) is produced whereas when the residence time of the catalyst is long, the polymer having relatively high catalyst efficiency is produced. In each reactor, the polymer having low catalyst efficiency and the polymer having high catalyst efficiency are produced. As a result, the formulation of the total polymers has a broad distribution. Even though an average residence time in each reactor is set to give 15% of a content of the propylene-ethylene copolymer (referring to as EPR) in the second step, the resulting block copolymer contains polymer grains having zero % of EPR content and polymer grains having about 100% of EPR content as extreme expression. Among them, the polymer grains having relatively high EPR content do not disperse in the homopolymer of propylene to result in fish-eyes in the product. As a result, it is considered to give inferior cold shortness and impact strength.
Various processes have been proposed to overcome the disadvantages in the continuous polymerization.
The processes using many reactors have been proposed in Japanese Examined Patent Publication No. 25585/1978 and Japanese Unexamined Patent Publication No. 53990/1974. In such processes, a block copolymer having physical properties similar to those of batch process can be obtained, however, it causes higher cost of construction of the apparatus depending upon increase of the reactors to be uneconomical and it causes complicated quality control of the product. It is preferable to reduce a number of the reactors.
The process adding an electron donor in the copolymerization of propylene and ethylene in the second step has been proposed in Japanese Examined Patent Publication No. 19542/1969 and Japanese Unexamined Patent Publication No. 115417/1980. Thus, the process has not been effective according to the studies.
The process for reducing a molecular weight of a propylene-ethylene copolymer in the second step has been proposed in Japanese Unexamined Patent Publication No. 61278/1974. The process causes inferior impact strength of the product according to the studies.